1. Field of the Invention
The present invention relates to a gas pressure generator and in particular to a gas pressure generator of the type used to generate a substantially constant pressure of gas in a ventilator, especially fresh gas in an anesthesia ventilator.
2. Description of the Prior Art
Anesthesia ventilators are well known and generally contain a so-called "closed" or "partially closed" respiratory circuit, dependent on whether all or some of the exhaled respiratory gases are reused. These circuits reduce the consumption of fresh gas, for example respiratory air enriched with oxygen or anesthetic gas. In order to compensate for gas losses during a breathing cycle, a supply of fresh gas is provided for introduction into the respiratory circuit. The constituents of the fresh gas are often supplied from a high pressure source, such as a gas bottle or a hospital's centralized high pressure system, and its pressure must be reduced to around 100 cm H.sub.2 O before being supplied to a patient.
It is also desirable that the fresh gas supply to the respiratory circuit be maintained at a substantially constant over-pressure, not least because the amount of fresh gas introduced can be more easily monitored if it is delivered at a substantially constant pressure. In order to obtain such a substantially constant pressure fresh gas source, it is usual to provide a source of fresh gas connectable to a gas pressure generator having a variable volume container with an output that is connectable to the respiratory circuit. The volume of the container is variable in a manner so that the input pressure of fresh gas can be reduced to a usable level and fresh gas at a substantially constant pressure can be supplied as an output.
Although referred to as a "constant" pressure source in reality the output pressure may be allowed to vary within limits (typically 10%-20% of the desired pressure) dependent on the circumstances of intended use in a manner known to those skilled in the art.
One known variable volume container is described in U.S. Pat. No. 3,824,902 and includes an expansion bellows made of soft plastic material attached between two rigid plates. One plate is spatially fixed and the other is operably connected to a shaft which, when rotated, collapses the container by means of springs. Through suitable choices of spring and spring attachment points the container can be made to collapse so that over a certain range of volumes the force exerted on the container is constant, thereby producing a constant pressure fresh gas output. Such a container, however, is mechanically complex which results in relatively high construction costs and likelihood of malfunction during use.
A further variable volume container is described in European Application 0 744 184 which also includes a collapsible bellows arrangement. This bellows is formed, at least in part, of an elastic material. Through careful choice of material and constructional configuration the bellows is made to produce, over a limited range of volume change, a constant pressure on the fresh gas contained in it as its volume reduces. Although a great simplification over the aforementioned container, a problem still remains that, as with the previously described bellows, the constant pressure gas can only be generated over a limited range of volume changes, which range tends to be removed from the zero volume condition.
This means that the container usually has a relatively large "dead volume" of fresh gas in it which is not removed during the breathing cycle of a patient. This may cause problems when a change in the constituents of the fresh gas is needed during an operation because a relatively large, often expensive, volume of gas has to be flushed out of the container. This is time consuming and fresh gas cannot be supplied, or is supplied but with an unknown constituency, to the patient during this time.